most of the time, you don't think about the little details of your life that subtly indicate your age. things such as: your handwriting. your music preferences. your internet habits. what you hang on your wall.
you fail to notice how, for example, your choice of underwear changes during the first three decades of life--probably because it doesn't all change at once. it's not like you went from taiwanese cartoon-print stretchy underpants to animal print overnight (not speaking specifically of personal underwear collections, of course). no. it happens gradually. purchase by purchase.
i heard somewhere that the human body completely replaces itself on a cellular level over the course of seven years. my stock of underwear completely replaces itself over the course of... i don't know. less than seven years. in a short enough time span that i'm beginning to think that it's one of the most accurate measurements of my progress as a human being.